In the 1890s realising that a generation of socialist and anarchist militants from the mid-19th century was passing away and their archives of writings and correspondence being destroyed, he concentrated his effort and a recent modest inheritance from his father on acquiring and rescuing such collections from destruction. He also made many interviews of veteran militants for posterity. He wrote biographies of many famous anarchists, including Mikhail Bakunin, Élisée Reclus, and Errico Malatesta. He also wrote a seven volume history of anarchism.

His extensive collection or archives was sold to the International Institute of Social History in Amsterdam in 1935. He lived continuously in Amsterdam from 1938 where he worked on cataloging the archive for the Institute. "The Nazis, apparently, were not aware of this fact",[1] so he died there suddenly from stomach cancer in 1944, without ever being harassed.